An FBI investigation into suspected security breaches involving Pentagon officials and Israel is unlikely to result in prosecution of senior figures following pressure from the White House, according to people familiar with the case.

The investigation has highlighted concerns that a small group of neo-conservatives in the Pentagon not only may have divulged classified information to Israel, but also tried to mount intelligence and foreign policy operations without informing the State Department and Central Intelligence Agency.

Analysts said that although the neo-conservative proponents of regime change in Iraq and Iran had fallen out of favour with the White House, the presidential election in November still afforded them protection.

The White House denied allegations of a cover-up. A spokesman said there was full support for the investigation.

Sources familiar with the investigation said the White House and John Ashcroft, the US attorney-general, had intervened to apply the brakes. “The White House is leaning on the FBI. Some people in the FBI are very upset, they think Ashcroft is playing politics with this,” a former intelligence official said.

Paul McNulty, the Virginia district attorney in charge of the probe, had been told to slow down, the sources said. Asked for comment, Mr McNulty’s office would only say that the investigation was continuing.

The investigations came to light last month, when officials confirmed reports that Lawrence Franklin, a mid-level analyst at the Pentagon, was the subject of an FBI inquiry into whether he passed classified information to an Israeli diplomat in Washington and to the American Israel Public Affairs Committee (Aipac), an influential lobby group.

Aipac had been under investigation for more than two years, a senior White House official said. Aipac and Israel deny involvement in espionage.

Stephen Green, author and investigative reporter, said the FBI had interviewed him about several prominent neo-conservatives, including Paul Wolfowitz, the deputy defence secretary, Douglas Feith, undersecretary for policy at the Pentagon, as well as former officials including Richard Perle, Stephen Bryen and Michael Ledeen.

The FBI is said to be looking back at investigations into alleged breaches of security involving Israel and current and former officials. None of the cases reached court. However, David Frum, former speechwriter for the president, said the investigators had found nothing serious and were about to drop the matter. He described what he called an anti-Israeli obsession among some parts of the administration who viewed Israel “not as the ally it is by law and treaty but as the source of all the trouble in the Middle East and the world”.

Mr Franklin, officials say, is suspected of providing Aipac with a draft national security presidential directive that proposed a tougher policy towards Iran.

In December 2001, Mr Franklin attended a meeting in Rome organised by Mr Ledeen with Manucher Ghorbanifar, an Iranian arms dealer with ties to Israel, and Antonio Martino, Italy’s defence minister, and Nicolo Pollari, head of Italian military intelligence.